Objectives Determine efficacy and modes of action for pertussis toxin immunomodulatory therapy in SIV infection. ABSTRACT:Pertussis toxin from the gram-negative bacterium Bordetella pertussis is an ADP-ribosylase that modifies Gi proteins in mammalian lymphocytes and inhibits their capacity to traffic from blood into lymphoid tissues. We used this compound to induce lymphocytosis in SIV infected rhesus macaques. Our objective was to evaluate an agent that modifies lymphocyte phenotype and reduces lymphocyte trafficking, for its effects on virus replication and pathogenesis in vivo. Pertussis toxin injected at 25 g/kg induced a transient lymphocytosis that peaked 3-8 days after administration and caused a rapid, transient decrease in the frequency of infectious cells in blood. Lymphocyte subsets were altered during the lymphocytosis interval and sustained changes in CD8+ T cell levels were noted at 53 days after pertussis toxin injection. In situ hybridization studies showed the effects of pertussis toxin on viral RNA distribution and microscopic anatomy of lymph nodes. Keywords AIDS, immunotherapy, biological response modifier, G proteins, ADP-ribosylase, lymphocytosis